1. Field of the Invention
The present invention relates to a method of removing NO.sub.x from effluent gases containing N.sub.2 and NO.sub.x, or N.sub.2, NO.sub.x and sulfur dioxide (SO.sub.2), or N.sub.2, NO.sub.x and O.sub.2 and/or CO.sub.2, and so forth.
2. Description of the Prior Art
Effluent gases from various boilers and heating furnaces contain N.sub.2, oxygen (O.sub.2), NO.sub.x, SO.sub.2 and so on, while exhaust gases from vehicles, such as automobiles generally contain N.sub.2, NO.sub.x, carbon monoxide (CO) and hydrocarbons (H.C.), wherein the NO.sub.x is present in concentrations which are usually higher than in effluent gases from heating furnaces and so on.
Each poisonous substance, i.e. NO.sub.x, SO.sub.2 and H.C. is a source of pollutant in the atmosphere, respectively.
Several methods of removing NO.sub.x have been well known, such as catalytic decomposition, catalytic reduction, oxidation-absorption or adsorption processes, etc. Particularly, a method of selective catalytic reduction of NO.sub.x with ammonia (NH.sub.3) is presently known as a superior process in connection with removing NO.sub.x from effluent gases of large boilers of power plants, heating or sintering furnaces in iron manufacture of heat-cracking furnaces of refineries and petrochemical factories.
Hereinafter, such plants and automobiles are referred to as fixed and movable sources of NO.sub.x, respectively.
Developmental research on catalysts with excellent activity, life and regeneration properties has continued with respect to this method. Nevertheless, no satisfactory catalyst has been developed by these efforts. Recently, several interesting patents disclosing methods of selectively reducing NO.sub.x with NH.sub.3 in the absence of a catalyst have been published.
In the oxidation-absorption methods, NO.sub.x is oxidized firstly to NO.sub.2 with an inorganic reagent, for example, chlorite in the liquid phase or with an oxygen or ozone (O.sub.3) containing gas in the liquid or vapor phase, in the presence or absence of a catalyst, while NO.sub.x is photochemically oxidized with a molecular oxygen containing gas. The formed NO.sub.2 is absorbed with an alkaline or acidic solution (aqueous nitric acid) to form salts or dilute nitric acid solutions, so that the extent of the application of this method is limited or narrow unless the resulting salts or dilute nitric acid solutions may be widely utilized. Alternatively, a number of methods of NO.sub.x removal have been found which utilize a metal complex salt or a metal chelate compound. These methods may be classified as a so-called wet-method as well as the oxidation-absorption method already mentioned. It is, in general, necessary in the wet-method to treat the accumulated solution without secondary pollution. If such a solution is not otherwise useful, the problem of treating the waste solution is very difficult and troublesome.
In the NO.sub.x adsorption method, active carbon and other adsorbents and the molecular sieve effect have been investigated, and yet such developmental research would take time.
Various catalysts have been reported, but no industrial scale NO.sub.x catalytic decomposition method is known.
In addition, various methods of physically decomposing NO.sub.x will be described as follows:
i. Method of irradiating a gas containing NO.sub.x, SO.sub.2, N.sub.2, O.sub.2 and so on with an electron beam;
One such method is described in Japanese Patent Application Laid-Open No. 93268/1974.
ii. Method of treating a NO.sub.x -containing gas with high frequency;
One such method is described in Japanese Patent Application Laid-Open No. 42471/1972.
iii. Method of treating a NO.sub.x -containing gas by electric discharge:
On such method is described in U.S. Application Ser. No. 151,396, filed June 9, 1971, corresponding to Japanese Patent Application Laid-Open No. 6974/1973, which shows a method of decomposing NO.sub.x characterized in that a dilute plasma of NO.sub.x is generated, and followed by contacting said plasma component with a large surface area solid supported heterogeneous catalyst, wherein no irreversible reaction may occur between said plasma and said solids. Further, Japanese Patent Application Laid-Open No. 96970/1974 shows apparatus for decomposing NO.sub.x in which NO.sub.x is passed in a discharge bed for decomposition, the bed having two opposite electrodes and being packed with articles, wherein discharge occurs between the electrodes or among those particles on electric supply. The NO.sub.x decomposing apparatus is characterized in that said discharge bed useful for decomposing NO.sub.x serves as a discharge bed for decomposing an oil by electric discharge and said oil generates a combustible gas such as hydrogen, etc. Alternatively, said apparatus is provided separately with a bed to supply a combustible gas to said discharge bed for decomposing NO.sub.x or generating a combustible gas, said gas being generated by decomposition of said oil by electric discharge.
In this method, it is intended that the NO.sub.x decomposition reaction and the catalytic oxidation occur simultaneously in the reaction vessel for enhancing the reaction effect. The particles may be, preferably, a metal having a catalytic action, such as Cu, Ni, Fe, etc. for the object reaction.
Japanese Patent Application Laid-Open No. 124871/1975 shows a method in which electric discharge is applied among packed metal lines, foils, rods and powders. Further, Japanese Patent Application Laid-Open No. 60494/1975 shows a method in which NO.sub.x reacts with atomic oxygen to form NO.sub.2 when corona discharge occurs in an atmosphere kept at the state of elevated temperature.
Japanese Patent Application Laid-Open No. 50271/1975 shows an electron bombardment apparatus for NO.sub.x removal, involving electron bombardment which comprises bombarding NO.sub.x molecules in the gas to be treated by an electron beam of a discharge zone, which results in elevating the energy level of NO.sub.x (NO, NO.sub.2, etc.) or which forms activated NO.sub.x, decomposing the activated NO.sub.x in an adsorption zone, and removing formed ozone with any suitable catalyst for decomposing ozone, characterized in that aluminium with an oxide film thereon is applied to the inner electrode of said discharge zone.
Japanese Patent Application Laid-Open No. 45768/1975 shows a method of removing NO.sub.x which includes a discharge between metal vapor generating bodies which are composed of a metal such as iron or other metal and are stretched between electrodes.
iv. Method of NO.sub.x removal by plasma:
Japanese Patent Application Laid-Open No. 122879/1974 shows a method of treating NO.sub.x contained in a gas which comprises heating the gas to a temperature of about 8000.degree. K by a plasma generating apparatus, generating plasma of NO.sub.x, atomizing NO.sub.x to N and O, followed by further treatment for reacting said atomic oxygen with any atom except atomic nitrogen to form an oxygen compound.
v. Method of NO.sub.x removal by ultraviolet ray:
One method is described in Japanese Patent Application Laid-Open No. 62366/1974.
On the other hand, methods of removal of poisonous substances (NO.sub.x, CO, HC, etc.) from exhaust gases from vehicles may be roughly divided as follows:
i. Controlling combustive conditions by improving the engine, for example, Rotary or C.V.C.C. engines. PA1 ii. Oxidizing or decomposing the poisonous substances in the presence of a catalyst. PA1 iii. Lowering the occurrence of the poisonous substances by employing fuel additives. PA1 i. The problem of item (i) of the catalytic decomposition method is common to this method. PA1 ii. The problem of item (ii) of the catalytic decomposition method, in any event, should be considered somewhat.
There are a number of catalysts useful for removing CO and HC; however, the need for a catalyst useful in removing NO.sub.x will be more increased in the near future. There are many problems to be solved in connection with such catalysts, such as activity and durability in particular, and mechanical strength, abrasive resistance, price and so forth. The main causes of the decrease of activity of such a catalyst are lead and high temperatures, so that the catalyst should be exchanged frequently. Recently, an opinion has been noted that secondary air pollution would occur, because the poisonous metal oxide catalyst is destroyed on travel and dust or fine powders thereof diffuse in or spread over the surrounding atmosphere.
As shown in the foregoing, various different methods have been disclosed with respect to removing NO.sub.x from fixed sources as well as from movable sources, however, no perfect or determinative method has been accomplished at the present time. Further, the removal of pollusive NO.sub.x on the order of p.p.m. is very expensive.
In brief, there are some important problems to be solved concerning those methods: